Serum phosphate levels are highly regulated within a narrow range. Both in vitro and in vivo studies have demonstrated that phosphate regulates the parathyroid gland independent of calcium and vitamin D. The addition of phosphate in the diet leads to an increase in parathyroid hormone (PTH) production and secretion. In normal humans, a circadian rhythm of (PTH) and phosphate has been observed. However, this rhythm is greatly reduced or absent in osteoporotic women. The intermittent administration of PTH may increase bone formation. Timed oral phosphate administration may drive PTH secretion and therefore promote bone formation in women with osteoporosis. This protocol is intended as a pilot study to test the hypothesis that a single evening dose of phosphate may re-establish the PTH circadian rhythm and thus promote bone formation. Five post-menopausal women with osteoporosis will be enrolled and studied for seven days. On the first day, a baseline 24 hour evaluation will be performed to determine circadian fluctuations in PTH, serum ionized calcium and phosphate, as well as bone metabolic markers. Following the baseline evaluation a daily (evening) dose of 1.2g phosphate will be given for 7 days. On the last day a 24 hour evaluation will be performed as before. The results of the two 24 hour evaluations will be compared.